The present invention relates generally to a zigzag sewing machine, and more particularly to a sewing machine capable of forming a line of stitches a constant predetermined distance away from the edge of a work fabric (hereinafter referred to as "fabric edge").
A zigzag sewing machine is adapted to sew stitches in a zigzag fashion by laterally jogging a sewing needle. Such a zigzag sewing machine capable of changing the lateral position of the needle, i.e., the needle position in a direction perpendicular to a feeding direction of the work fabric, can be readily provided with a fabric-edge tracing function for forming stitches along the fabric edge, by adding simple devices such as a sensor for detecting the fabric edge.
In the sewing machine having the fabric-edge tracing function indicated above, however, the line of stitches formed cannot always exactly follow the fabric edge, and may sometimes suffer from a sharp bend, particularly where the work fabric does not have a distinctly defined straight or curved edge without unwoven threads or fibers. The unwoven threads or other projections from the fabric edge affect the output signal produced from the fabric-edge sensor, as if the fabric edge were displaced in the lateral jogging direction of the needle. In this case, the lateral position of the needle is changed according to the output signal which does not correctly reflect the position of the fabric edge. Such an erroneous detection of the fabric edge frequently occurs when the work fabric consists of thick fibers, as in the case of edge-trace sewing operations on jeans. A similar inconvenience may take place in other cases, for example, where the stitching operation is effected on a workpiece which bears tacking pins.
In the light of the above inconveniences, the assignee of the present application proposed a sewing machine having a fabric-edge tracing function as disclosed in Japanese Patent Application No. 60-209564. This sewing machine includes (a) a feed mechanism for feeding a workpiece, (b) a needle-bar oscillator which supports a needle bar having a sewing needle fixed to its lower end, such that the needle bar is endwise reciprocable, and which is supported by a machine frame such that the needle bar is laterally joggable in a lateral direction substantially perpendicular to a direction of feeding of the workpiece by the feed mechanism, (c) a fabric sensor having a light emitting portion and a light receiving portion for detecting the edge of the workpiece which extends in the feeding direction, (d) a reflecting surface disposed on a work bed of the machine, in the vicinity of a lowered position of the needle, for reflecting a radiation emitted by the light emitting portion, toward the light receiving portion, (e) bight-signal generator means for generating a bight signal representative of a distance from the edge of the workpiece, according a detection signal from the fabric sensor, and (f) a bight control device responsive to the bight signal, for controlling an amount of lateral jogging of the needle. In this sewing machine, the needle position is displaced by the bight signal generated from the bight-signal generator means, at least when the amount of variation in the edge position of the workpiece between the moments of formation of two successive stitches by endwise reciprocating movements of the needle exceeds a predetermined value. The amount of displacement of the needle position is smaller than the amount of variation in the edge position of the workpiece. According to this proposed arrangement, the needle position is not so sensitively changed due to the presence of unwoven fibers or other projections from the fabric edge, whereby a conventionally experienced sharp bend of the stitching line can be avoided.